Gonolobus cthulhui (Apocynaceae), a new species from Oaxaca, Mexico

Background and Aims: In Mexico, the genus Gonolobus is currently represented by 43 species. Gonolobus has been studied for some regional floras and taxonomic novelties are continuously being described. In this context, during the review of the herbarium material and collections made in the state of Oaxaca, a specimen that failed to be assigned to any described Gonolobus in Mexico and Mesoamerica was found. Therefore, the objective of this work was to describe a new species of Gonolobus from Oaxaca. Methods: Eleven herbaria and five virtual collections were revised, and several field trips to Oaxaca were conducted in 2018 to collect specimens. The studied material was compared with other species with similar morphology, a comparative table was prepared, and a distribution map included. The conservation status of the species based on field observations according to the IUCN Red List criteria is presented. Key results: A new species of Gonolobus from southeastern Oaxaca, Mexico, with large green corollas, is described herein. A description and images of the new species are provided. Gonolobus cthulhui is morphologically similar to G. leianthus and G. pancololote, but differs by its black-tipped and widely ovate to suborbicular sepals, pubescent diagonal pattern on the corolla lobes, and thinner and discontinuous faucal annulus. An Endangered (EN) conservation category is suggested for this species. Conclusions: This discovery highlights Oaxaca as the second state, after Chiapas, with most species of Gonolobus (19 spp.), and Mexico as a center of diversity for the genus, with 44 species recorded, of which 52.2% are endemic to the country.